It is known that doping can alter organic semiconductors with regard to their electrical properties, in particular their electrical conductivity, as is also the case for inorganic semiconductors such as silicon semiconductors.
In this case, generation of charge carriers in a matrix material increases the initially quite low conductivity and, depending on the type of the dopant used, achieves a change in the Fermi level of the semiconductor. Doping leads to an increase in the conductivity of the charge transport layer, which reduces resistance losses, and leads to an improved transition of the charge carriers between contacts and organic layer.
For the doping of such organic semiconductors, strong electron acceptors such as tetracyanoquinonedimethane (TCNQ) or 2,3,5,6-tetrafluorotetracyano-1,4-benzoquinone-dimethane (F4-TCNQ) have become known; see M. Pfeiffer, A. Beyer, T. Fritz, K. Leo, Appl. Phys. Lett., 73 (22), 3202-3204 (1998) and J. Blochwitz, M. Pfeiffer, T. Fritz, K. Leo, Appl. Phys. Lett., 73 (6), 729-732 (1998). As a result of electron transfer processes in electron donor-like base materials (hole transport materials), these generate what are known as holes, the number and mobility of which more or less significantly alter the conductivity of the matrix material.
Known matrix materials are, for example, starburst compounds such as 4,4′,4″-tris(diphenylamino)triphenylamine (TDATA), 4,4′,4″-tris(3-methylphenylphenylamino)-triphenylamine (m-MTDATA) and N,N,N′,N′-tetrakis(4-methoxyphenyl)benzidine (MeO-TPD).
The chemical structure of the abovementioned known matrix materials is shown below:
 m-MTDATA  TDATA  MeO-TPD
However, these compounds are thermally unstable, i.e. they have a low glass transition temperature and tend to crystallization at low temperatures, which leads ultimately to unstable electronic components.
The glass transition temperature is regarded as being the temperature at which motion of the molecules in the event of rapid cooling of the material from the melt is no longer possible for kinetic reasons, and thermodynamic parameters such as the heat capacity or the coefficient of expansion suddenly change from typical liquid values to typical solid values. The thermal stability of the matrix material is of significance especially for morphological reasons when organic semiconductor materials are used with such matrix materials, in order to prevent the formation of roughness at elevated operating temperatures in the customary layer structure of such semiconductor materials. Furthermore, the thermal stability is of significance, in order to restrict the diffusion of the dopant within the matrix material.
The prior art also discloses thermally stable matrix materials such as 2,2′,7,7′-tetrakis(N,N-diphenylamino)-9,9′-spirobifluorene (spiro-TAD) which, however, owing to the position of the energy level of their highest occupied molecular orbitals (HOMOs), cannot be doped.
